Die seals are typically used to protect the circuitry inside the die seal from stress induced damage caused by the packaging process and harmful impurities such as mobile ions and moisture.
Since the die seal contains all the metal layers used in the IC device process flow, it serves as a good conducting path. For this reason, sometimes, the die seal is used as a ground bus, with ohmic contact to the semiconductor substrate. Though this provides a good conducting path for all of the metal layers, it can transport substrate noise from one region of a chip with high noise, such as s digital circuit block, to another region on the same die that requires very low noise, such as a region containing analog circuitry.
One prior art method for reducing noise coupling through the die seal is to cut the die seal into several portions. Though this alleviates the noise coupling problem, contaminants can enter through the gaps in the die seal, damaging the internal circuitry.
Another prior art method is to insert a polysilicon layer and gate oxide between the first layer of interconnects and the substrate. Though this isolates the gate from substrate noise, contaminants can enter through the polysilicon layer and the gate oxide layer, damaging the internal circuitry.
Accordingly, there is a need for a die seal and a method for forming a die seal that will provide an effective seal against contaminants and that will reduce noise coupling between different regions in the integrated circuit device.